The Islamic State-claimed bombing of a Catholic cathedral that killed 20 in the Philippines' restive south put fresh pressure Monday on peace efforts aimed at ending decades of separatist violence.

Philippine Air Force personnel assist their wounded comrade on a stretcher, who was one of the victims of the twin bombings, as he arrived along with other wounded victims aboard a military C-295 transport plane on January 27.//EPA-EFE

Two explosions tore through the cathedral, shattering pews and windows, on the Muslim-majority island of Jolo, killing worshippers at Sunday mass and security forces in one the nation's worst bombings in years.

Experts voiced concern on Monday over the impact the attack would have on a decades-long push for peace that culminated last week in voters approving expanded Muslim self-rule in the south.

The vote was the result of negotiations started in the 1990s with the nation's largest rebel group, the Moro Islamic Liberation Front (MILF), and will give it considerable power over the so-called Bangsamoro region.

The IS claim, in a formal communique, said two suicide bombers had detonated explosive belts, according to the SITE Intelligence Group, which monitors jihadist activities.

But a military report said the second bomb was left in the utility box of a motorcycle in the parking area outside the church. Police said they believe the explosives were detonated remotely, but did elaborate.

Despite the contradictions, authorities have not ruled out IS involvement.

Jihadist factions aligned with the group -- including the notorious Abu Sayyaf kidnap-for-ransom group -- which were not part of the peace process, are prime suspects in the bombing.

The remote island of Jolo is a base of the group, which is blamed for deadly bombings, including an attack on a ferry in Manila Bay in 2004 that claimed 116 lives in the country's deadliest terror assault.

- 'Opportunities squandered' -

"This is a big challenge for the Bangsamoro government," said Rommel Banlaoi, chair of the Philippine Institute for Peace, Violence and Terrorism Research.

The former rebels need to show they will be able to pull the region toward peace in order to attract much-needed investment to alleviate poverty and counter extremism, he told AFP.

"MILF needs to prove it can make a difference... the gravity of the problem faced by MILF is wow, so overwhelming," he added.

The church attack came despite President Rodrigo Duterte putting the southern Philippines under martial rule after pro-IS militants seized the southern city of Marawi in May 2017.

Government officials have argued that martial rule, which gives authorities extra powers, has been effective in taming the perpetually restive region.

Meanwhile, condolences poured in from around the world for the victims at the cathedral, which has been repeatedly targeted by grenade attacks that did not prove fatal.

Pope Francis, speaking in Panama, expressed his "strongest reprobation" for the violence. Once again, he said, "the Christian community has been plunged into mourning."

But experts were also worried about how the attack would impact the hopes for new development in the region, which were spurred by the self-rule vote victory.